1. Technical Field
The invention relates to an optical head for scanning an optical recording carrier having an information layer and a transparent layer, comprising a radiation source for generating a radiation beam and an objective system for converging the radiation beam through the transparent layer to a focus on the beam information layer, an aberration detector for detecting an optical aberration in the radiation beam, an aberration compensator arranged in the optical path between the radiation source and the objective system, and a control circuit connected to an output of the aberration detector for controlling the aberration compensator.
The information stored in an optical record carrier is arranged in tracks in the information layer of the record carrier. The information is written, read or erased by means of a focussed radiation beam that follows the track. The position of the focus is kept in the plane of the information layer by means of a focus servo that controls the axial position of the objective lens used for focussing the radiation beam. A second servo system controls the transverse position of the focus in order to keep the focus on the track being scanned. The transverse direction is the direction in the plane of the information layer perpendicular to the direction of the track. The second servo system causes the objective lens to move in the transverse direction, i.e. in a direction perpendicular to its optical axis, thereby moving the focus also in the transverse direction. On a disk-shaped record carrier the transverse direction corresponds to the radial direction; therefore the second servo system is also called the radial tracking servo.
The trend of increasing information density on optical record carriers requires a commensurate decrease in the size of the focus of the radiation beam formed on the information layer. A smaller focus can be realized by increasing the numerical aperture of the radiation beam incident on the record carrier. However, an increase of the numerical aperture increases the susceptibility of the optical system in the head to optical aberrations. One of the aberrations is coma, caused by the transparent layer of the record carrier when it is not perpendicular to the principal ray of the radiation beam incident on the record carrier. Such non-perpendicular incidence of the radiation beam on the record carrier, generally referred to as tilt, may be caused by warping of the record carrier. Optical heads having a high numerical aperture require compensation of the coma caused by the tilt in order to scan the information layer of the record carrier properly.
2. Related Art
European Patent Application No. 0 745 980 shows an optical head provided with a tilt compensator. The tilt compensator is an electrostriction device arranged in the optical path between the radiation source and the objective system. The known head is also provided with a tilt detector for detecting tilt of the record carrier. The output signal of the tilt detector is used to control the tilt compensator, which introduces a wave front aberration in the radiation beam that compensates the coma caused by the tilted record carrier. It is a disadvantage of the known device, that the aberration compensation does not operate properly when the optical head is following a track.
It is an object of the invention to provide an optical head that has a good aberration compensation independent of the tracking of the focus.
This object is achieved if, according to the invention, the optical head of the preamble comprises a position detector for determining a transverse position of the objective system and that the control circuit is connected to an output of the position detector. The invention is based on the insight that the aberration introduced by the aberration compensator causes other aberrations in the radiation beam when the objective system is displaced in a transverse direction. The magnitude of the other aberrations depends both on the amount of aberration introduced by the aberration compensator and on the displacement of the objective system. Therefore, the aberration compensator should not only be controlled by the output of the tilt detector but also by the output of a position detector for determining the transverse position of the objective system, in order to take the transverse position into account when controlling the aberration compensator.
Preferably, the aberration compensator introduces an amount of a first optical aberration in the radiation beam of the type as measured by the aberration detector and an amount of a second optical aberration of a lower radial order than the first aberration. If not otherwise stated, all aberrations in a cross-section of the radiation beam are centred on the optical axis of the radiation beam at the cross-section. Hence, the first aberration is centred on the optical axis, whereas the objective system may be off-centre when tracking. The aberration compensator should also introduce an aberration that is off-centre to the same extent as the objective system. Such an off-centre aberration may be described as a linear combination of a first aberration of the same type as the off-centre aberration but centred on the optical axis and a centred second aberration of a lower radial order than the first aberration. This can be explained as follows. The second aberration is the difference between the off-centre aberration and the same centred aberration. For a small decentring, the second aberration proportional to is the derivative of the aberration in the radial direction of the decentring. Hence, the radial order of the second aberration is at least one order lower than that of the first aberration.
The aberration compensator may introduce the aberrations in the radiation beam in two ways, as a decentred first aberration or as a combination of a centred first aberration and a centred second aberration. In the first case, the amount of the decentred first aberration may be controlled by the output signal of the aberration detector and the amount of decentring by the output of the position detector. Alternatively, the amount of the decentred first aberration is controlled by a combination of the output signals of the aberration detector and the position detector, the magnitude of the decentring being fixed, and the sign of the decentring is controlled by the sign of the output signal of the position detector. In the second case, the amount of the first aberration may be controlled by the output signal of the aberration detector and the amount of the second aberration by the output signal of the position detector.
The aberration detector may be a tilt detector for detecting tilt of the record carrier, and the aberration compensator introduces coma as a first aberration in the radiation beam, which compensates the coma caused by the tilt. The astigmatism caused by the off-centre objective system is preferably compensated by astigmatism introduced as a second aberration by the aberration compensator.
In a preferred embodiment of the optical head, the aberration compensator introduces astigmatism by means of two similar, transversely displaced electrode structures. Each of these structures introduces coma in the radiation beam. Since this coma is off-centre, it can be described as a combination of centred coma and astigmatism. If the objective system is off-centre in one direction, one set of electrode structures will be energized; if the objective system is offset in the opposite direction, the other set of electrodes will be energized.
In a special embodiment the aberration compensator comprises the series of strip electrodes in a symmetrical arrangement. The strips may have a curved shape. Any combination of coma and astigmatism may be obtained by setting each of the strips at a specific voltage. In a preferred embodiment the voltages are formed by a series arrangement of resistors having taps, the strip electrodes being connected to the taps. The desired voltage distribution over the strips can be achieved by providing the voltages to the end taps and to least one intermediate tap. The electrode structure is preferably etched in a transparent conductive layer. The same conductive layer may be used to form the series arrangement of resistors.
A further aspect of the invention relates to a device for scanning an optical record carrier having an information layer and a transparent layer, comprising an optical head according to any of the previous claims and an information processing unit for error correction.